Communication systems are known to comprise a plurality of subscriber units that communicate with one or more base units via radio frequency channels. One such communication system is a two-way cable communication system. In a two-way cable communication system, the subscriber units are fixed transceivers that are located in the residences of system subscribers and are connected to a base unit through existing cable television infrastructure equipment. Each subscriber unit conveys information to the base unit at an uplink frequency and the base unit conveys information to each subscriber unit at a downlink frequency. The uplink frequency and the downlink frequency together comprise a radio frequency channel.
During the transmission of a communication signal from a subscriber unit to the base unit, interference (typically known as ingress interference) may be coupled onto the communication signal due to radiation of interfering signals at, or near, the uplink frequency from a nearby wireless transmitter or due to conduction of interfering signals at, or near, the uplink frequency from electronic devices, such as televisions or video cassette recorders (VCRs), directly connected to the cable infrastructure equipment. The majority of ingress interference is typically generated at frequencies located at the lower end of the cable communication uplink frequency band (e.g., 5-15 MHz of the 5-42 MHz band). When present, the ingress interference degrades the signal quality of the communication signal received by the base unit.
Two known techniques for mitigating the effects of ingress interference in a cable communication system are high frequency transmission and source isolation. The high frequency transmission technique allows uplink transmissions to occur only at uplink frequencies residing at the upper end of the cable communication uplink frequency band (e.g., 20-42 MHz) and out of the typical frequency band of the ingress interference. Thus, this technique is spectrally inefficient because it allows only for uplink communications in approximately the upper one-half of the available frequency range, thereby significantly limiting system capacity. The source isolation technique allows uplink transmissions at frequencies throughout the complete uplink frequency band; however, it burdens the communication system by requiring the base unit to first locate, and then isolate, sources of ingress interference when poor uplink signal quality is detected. In addition, isolation of the interference sources (e.g., by disconnecting uplink service to a home or a neighborhood) disrupts two-way communication service to the subscribers located in the geographical areas of the ingress interference sources.
Therefore, a need exists for a method and apparatus that mitigate interference in a communication system without detrimentally impacting system capacity and without creating any disruptions in service.